When do you know that your O-2 sensor is bad, I am getting a check engine light every once in awhile. I cannot check it due to the fact that I am stationed in Germany and no one at the dealer wants to admit they speak english. The car is running great, maybe a little down on MPG's, but I just chalked that up to running at 120MPH on the autobahn. Is there another way to check engine light to find out what the codes are?